The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
A photoplethysmography (PPG) sensor may be utilized to detect the volumetric change in blood vessels. A PPG sensor usually consists of a light source, typically a light-emitting diode (LED), and a light-sensitive sensor, typically a photodiode. Blood which passes between the light source and sensor will modulate a characteristics of the tissue along the light path between the two, resulting in a detectible deviation in the current produced by the photodiode. From this signal and applying various algorithms, a heart rate estimate can be determined.
Typical PPG technologies rely on emitting a single wavelength of green, red or infra-red (IR) light from an LED. Many wearable PPG devices use green light, as with green light the hemoglobin absorption of light is up to 20 times greater at green wavelengths than at IR wavelengths. However, red or IR light devices may be more efficient and use less power, and photodiodes tuned to these wavelengths may be more responsive. PPG technology suffers from severely reduced accuracy when the user is performing high-motion activities or certain activities which contort the wrist and thus affect the dynamics of blood flow within the wrist. The received light is modulated by these movements at an order of magnitude much greater than the desired cardiac signal. Low signal quality in PPG signals also can be caused by characteristics of the local area that is sensed. For instance, signal quality can vary greatly even if a wrist-worn PPG sensor is moved only a few millimeters up or down the wrist or signal quality can also vary due to variations in the wavelength being emitted. In addition, during motion, certain orientations of wrist-worn PPG devices are subject to more motion and, therefore, greater degradation of the PPG signal due to such motion.